Electrodeless discharge lamps comprise a bulb and an induction coil. The bulb has a noble gas and a light-emitting material both sealed therein. Examples of electrodeless fluorescent lamps include, for instance, lamps in which an induction field, generated by a high-frequency current flowing through an induction coil, causes discharge in a bulb, exciting thereby mercury as a light-emitting material. The ultraviolet radiation from the excited mercury atoms strikes then a phosphor, whereupon the ultraviolet radiation is converted into visible light. The structure of such electrodeless discharge lamps comprises no electrodes inside the lamp. Therefore, such lamps are not subject to defective lightning caused by electrode deterioration, and boast a longer life than ordinary fluorescent lamps.
A bismuth-indium amalgam is used as the supply source of mercury vapor in the electrodeless discharge lamps disclosed in Japanese Patent Application Laid-open No. H7-272688 and Japanese Utility Model Application Laid-open No. H6-5006. Such amalgams are advantageous in that they afford high light output over a wide range of surrounding temperature. However, a high amalgam temperature is required in order to release the necessary mercury vapor for realizing high light output, and it takes time to reach the required temperature. Long rise times are thus a shortcoming of such amalgams. Some results show that, when using a bismuth-indium amalgam, it takes about one minute to secure 60% light output relative to light output during stable lighting.
By contrast, Japanese Patent Application Laid-open No. 2001-325920 (hereinafter, Patent document 1) discloses an electrodeless discharge lamp in which pure mercury (mercury droplets) are used instead of an amalgam, with a view to shortening the rise time. The above document discloses that 50% of maximum output is reached within 2 to 3 seconds after starting the lamp. The reason for this is that mercury droplets afford high mercury vapor pressure at a lower temperature than in the case of an amalgam, so that the time that it takes to reach a required temperature is shorter. However, bulb temperature rises when input power relative to bulb volume is substantial, and/or when the surrounding temperature is high. The mercury vapor pressure becomes then excessively high as a result, causing light output to drop. In the above document the mercury vapor pressure is controlled to an appropriate value by providing a protrusion, as a coldest spot, in the bulb.
When using mercury droplets as the form in which mercury is sealed in the lamp, it is difficult to manage the amount of mercury sealed in, and thus mercury may become sealed in the lamp in an amount greater than required. The amount of mercury sealed in the lamp must be as small as possible, both in terms of environmental protection and in order to prevent light output blocking on account of adhesion to the phosphor surface. To address these shortcomings, Japanese Patent Application Laid-open No. 2005-346983 (hereinafter, Patent document 2), for instance, discloses the features of providing a protrusion, as a coldest spot, on a bulb, and using a Zn—Hg amalgam as the form in which mercury is sealed in the lamp.
As described above, Patent documents 1 and 2 disclose known methods of obtaining high light output by providing a protrusion on a bulb and by controlling mercury vapor pressure to an appropriate value. When the lit bulb is facing downwards (namely at an orientation such that the base disposed in the bulb is arranged facing up), the protrusion stands at the location on the surface of the bulb that is at a lowest temperature, i.e. the protrusion becomes the coldest spot. The mercury vapor pressure in the bulb is determined by the temperature of the coldest spot, while the light output of the lamp is governed by the mercury vapor pressure in the bulb. Therefore, the mercury vapor pressure in the bulb can be optimized, and hence the light output of the lamp can be optimized, by providing a protrusion in the bulb and by controlling the temperature of the coldest spot.
The protrusion becomes thus the coldest spot when the bulb is lit facing downward. Therefore, the temperature of the coldest spot can be regulated (controlled) by modifying the diameter and/or height of the protrusion. When the bulb is lit facing upward (namely at an orientation such that the base disposed in the bulb is arranged facing up), however, the temperature of the protrusion rises by virtue of its being disposed at the top of the bulb, so that the protrusion becomes no longer the coldest spot. When the bulb is lit facing upward, therefore, light output may drop, since the temperature of the coldest spot cannot be now controlled by way of the protrusion. Further, the output may vary depending on the lighting direction of the lamp.